Windows provide the primary means to control air flow in most homes.
People open windows to provide fresh air, ventilate odors and smoke,
dissipate heat and moisture, and create air movement on hot days. While
exhaust fans and central air systems can mechanically ventilate a room,
opening a room to the outdoors is perceived as more direct and natural.

Guidelines for Providing Fresh Air

Place operable windows in all rooms to give occupants opportunity for fresh air.

Provide cross-ventilation by placing window openings on opposite walls in line with the prevailing winds.

Use casement windows to direct and control ventilation.

Use operable skylights or roof windows to enhance ventilation.

Use landscape elements to direct breezes.

Connecting Indoor and Outdoor Space

In order to ensure that all residences have access to the healthful
aspects of natural ventilation, state or local building codes commonly
regulate the minimum size of the ventilation opening in a window and the
egress opening. Typically, codes require that about 5% of the
floor area of a "living area," such as a bedroom or living room, be
provided in ventilation area. These regulated areas should be carefully
checked before sizing or replacing a window.

The potential value of natural ventilation as an energy efficiency
strategy depends on climate and lifestyle. In a mild climate, there may
be many hours during the year when outdoor air can be used to improve
comfort and save energy for air conditioning. In climates with severe
summers and/or winters or in dusty, noisy, or humid locations, the value
of natural ventilation may be limited. To the extent that natural
ventilation requires occupants to open and close windows, the lifestyle
of the occupant may also be a factor. Finally, security concerns may
limit the opportunities to provide natural ventilation.

An alternative approach that provides a steady amount of outdoor air is
to incorporate a small ventilation element into the frame of the window.
This passive approach has been used in the northwest United States to
meet state code requirements for a minimum amount of outdoor air in new,
tightly built houses that do not use mechanical ventilation. These
"trickle ventilators" have been widely used in European houses. The
slots go through the window frame, normally on the top or bottom, with
screens and flaps that keep out bugs and rain. They can be adjusted by
occupants to control the amount of air flow. The peak amount of air
exchange can be controlled in each room by properly sizing the
ventilators in each window.

Improving Ventilation
Ventilation is maximized by providing for cross-ventilation of as many
spaces in the house as is practical. In normal wind conditions, the side
of a building facing the wind will have a zone of positive pressure and
the opposite side will have a zone of negative pressure.
By providing adequate ventilation openings on these two sides of the
house, a positive flow of air through the interior, from positive to
negative pressure, is encouraged. Of course, the interior layout of the
house must permit the air to flow through, and interior doors in the
ventilation path must remain open.

Ventilation effectiveness depends on wind speed, the angle at which the
wind strikes the window, and the location and size of the window. A room
with a single opening will have only 12–23% of the wind
velocity. This improves up to 51% if windows are located on
adjacent walls and as much as 65% of the outside air velocity can
be reached with windows on opposite walls.

If windows cannot be located on opposing walls, high- and low-pressure
areas can be induced with the use of casements. Of all
window types, casements provide the most control of ventilation
direction and intensity. Because the sash can be opened into an air
stream, breezes that would otherwise pass by can be directed into the
room. Window types in which the sash remains flush with the wall
ventilate well only with direct pressure differences across the window.
In addition, as noted previously, virtually the entire window area of
casement units can be opened, while sliders are limited to less than
half of the area.

Even without external winds, double-hung windows can sometimes provide
natural ventilation caused by stratified air flow within a room. Cooler
fresh air enters at the bottom opening, while hotter air at the ceiling
level is allowed to exit through the top opening. The taller the windows
and the higher the ceiling, the more pronounced is this effect.

Operable skylights or roof windows can aid overall ventilation in a
house significantly by creating a similar thermal chimney effect,
letting hot air escape from the ceiling level where it accumulates and
causing cooler air to be drawn in through lower windows.

Trees, shrubs, exterior walls, and earth berms can all divert wind
patterns. These elements can be used to some extent to funnel or direct
breezes through a house. Conversely, if these elements are placed
without regard for prevailing wind patterns, they can act as
obstructions to natural ventilation.